I've read both this book and Dave Baum's Definitive Guide and both are excellent. I enjoyed Baum's book more because it caters to an older audience: Baum seems to be writing to an older reader, there's an emphasis on the programming aspect of Mindstorms, and there are more semi-advanced projects.
Erwin's book is a thousand times better for children. The brilliant full-color pictures blow away Baum's black and white book. Erwin is a genuinely interesting author and obviously loves teaching.. Anyone who wants to teach a class with Mindstorms should take a look at the book, because it's full of great "this didn't work, but THIS did" anecdotes.
Ultimately, this never really goes above that level. Teachers and younger students should purchase this book, because it's a beautiful guide and will inspire quite a few youngsters. For the robotics engineers, programmers, and older geeks who wish that they'd had Mindstorms as a child, check out Baum's Definitive Guide - it's definitely more of what I was looking for.